The present invention relates to a magnetic head coil suitable for recording an information signal at a high speed, a magnetic head using it, and a magneto-optical recording apparatus.
A conventionally known magneto-optical recording apparatus applies a magnetic field modulated by an information signal to a magneto-optical recording medium such as a magneto-optical disk, and irradiates the medium with light to record an information signal. This magneto-optical recording apparatus comprises a magnetic head for applying a magnetic field. The magnetic head may be one of various types of heads. For example, FIG. 17 is a perspective view showing a magnetic head disclosed in Japanese Laid-Open Patent Application No. 4-74335, and FIG. 18 is a sectional view showing the magnetic head.
Reference numeral 50 denotes a flat coil component (to be referred to as a coil hereinafter) formed from a flexible printed wiring board; and 51, a core made of a magnetic material such as ferrite. The coil 50 is constituted by a flexible base 52 made of polyimide or polyester, a spiral coil pattern 53 serving as a conductor pattern made of a copper foil formed on the base 52, and terminals 54a and 54b. The coil 50 is bonded to the core 51 with an adhesive 55.
The terminals 54a and 54b of the coil 50 are connected to the magnetic head drive circuit of a magneto-optical recording apparatus. The magneto-optical recording apparatus comprises an optical head. To record an information signal, the optical head irradiates the magnetic recording layer of a magneto-optical recording medium with a laser beam so as to converge the laser beam to a small light spot. At the same time, the magnetic head drive circuit supplies a current to the coil pattern 53 to generate a magnetic field modulated by an information signal from the center of the coil pattern 53, and vertically applies the magnetic field to the laser beam irradiation position of the magnetic recording layer.
Conventionally, like this prior art, only a conductor pattern serving as a path for positively supplying a current, i.e., a conductor pattern necessary for an electrical function is formed on components using conductor patterns including a flat coil component for a magnetic head.
In recent years, as demands have arisen for a higher information signal recording speed, the flat coil component used in the magnetic head must be downsized. Along with this, the dimensional precision and flatness of the flat coil component must be increased to adjust the relative position to the optical head and the distance from the magneto-optical recording medium at higher precision. The magnetic field must be accurately, efficiently applied to the light spot position on the magnetic recording layer of the magneto-optical recording medium. However, the above-described flat coil is low in rigidity and mechanical strength, readily deforms in manufacturing a magnetic head, and is difficult to be adjusted to an accurate position. Thus, the above demands cannot be met. This problem will be explained in detail.
To more efficiently generate a magnetic field in the above magnetic head, the coil pattern 53 must be formed very close to the core 51. For this purpose, the base 52 must be as thin as possible. To efficiently apply a magnetic field to the magneto-optical recording medium, the surface of the coil 50 must be brought very close to the magneto-optical recording medium.
Although not described in the above reference, the base 52 constituting the coil 50 is made of a 20-xcexcm thick polyimide sheet. Since the thin resin material sheet is very flexible, the coil 50 is insufficient in rigidity, posing the following problem in manufacturing a magnetic head.
More specifically, in bonding the coil 50 and the core 51, the coil 50 cannot resist an operating force and readily deforms, e.g., bends at a portion where no coil pattern 53 is formed. As a result, the attaching position of the coil 50 is not accurately determined, causing an error. The relative position to the optical head deviates, so an information signal cannot be normally recorded.
A conductor pattern for connecting the coil pattern to the terminal 54b is formed to protrude from the base 52 on a surface of the coil 50 facing the core 51. Thus, the surface of the coil 50 facing the core 51 is not flat. In bonding the coil 50 to the core 51, part of a surface of the coil 50 facing the magneto-optical disk readily deforms, e.g., protrudes or inclines. This inhibits the surface of the coil 50 facing the magneto-optical recording medium from coming very close to the magneto-optical recording medium so as to efficiently apply a magnetic field.
To increase the information signal recording speed, the magnetic field modulation frequency must be increased. However, the RF loss on the core 51 and coil pattern 53 increases in almost proportion to the modulation frequency, so that the temperature of the magnetic head rises. The magnetic material such as ferrite forming the core 51 decreases in saturation flux density Bs along with the temperature rise. As the magnetic field modulation frequency increases, the saturation flux density Bs of ferrite forming the core 51 decreases to be equal to the internal flux density of the core 51. If the magnetic field modulation frequency further increases, the internal flux density of the core 51 decreases together with the saturation flux density Bs, and the strength of a magnetic field generated by the magnetic head also decreases. As a result, a magnetic field applied to the magneto-optical recording medium weakens, failing to record an information signal.
If the temperature of the magnetic head exceeds the heat resistance limit of its building member, deformation or electrical insulation failure may occur.
Under these circumstances, an increase in modulation frequency is limited, and the information signal recording speed cannot be further increased.
In the present invention, a flat coil (to be referred to as a coil hereinafter) for a magnetic head is made up of at least a coil pattern serving as a conductor pattern made of a conductive material film, and a terminal for supplying a current to the coil pattern. The coil pattern is a spiral conductor pattern capable of supplying a current so as to flow around the magnetic field generation center. In the present invention, a region where this coil pattern is formed is defined as an xe2x80x9ceffective regionxe2x80x9d where an effective current contributing to generation of a magnetic field can be supplied. A region outside the coil pattern where at least the conductor pattern capable of supplying a current so as to flow around the magnetic field generation center is not formed is defined as an xe2x80x9cineffective regionxe2x80x9d. In the following description, conductor patterns formed in the ineffective region except for a conductor pattern serving as a current supply path to the coil pattern, such as a conductor pattern for connecting terminals to each other and a terminal to the coil pattern, will be referred to as a xe2x80x9cdummy patternxe2x80x9d.
The present invention has been made to overcome the conventional drawbacks, and has as its object to provide a flat coil for a magnetic head in which a conductor pattern is formed in the ineffective region, and a conductor occupation ratio R (ratio of the total area of all conductor patterns formed from a conductive material film in a given region, to the total area of the region) is defined within a predetermined range in accordance with the distance from the coil pattern, thereby improving the mechanical strength, flatness, and dimensional precision without degrading the electrical characteristics of the coil, a magnetic head using the flat coil, and a magneto-optical recording apparatus.
The present inventors have made extensive studies to find that the above problem can be solved when, letting S be the distance from the outer edge of the coil pattern (outer edge of the effective region), P be the pitch (or minimum value when the pitch is not constant) of the coil pattern, and R be the conductor occupation ratio, the ineffective region outside the effective region is divided into a plurality of regions on the basis of the distance S, conductor patterns are laid out in the respective regions so as to simultaneously satisfy inequalities 1, 2, and 3, and the conductor pattern in a first region A1 does not form any closed loop:
Inequality 1: 0xe2x89xa6Rxe2x89xa60.3 in the first region A1 where
0xe2x89xa6Sxe2x89xa61.5P
Inequality 2: 0xe2x89xa6Rxe2x89xa60.8 in a second region A2 where
1.5P less than Sxe2x89xa66.0P
Inequality 3: 0.3 less than Rxe2x89xa61 in a third region A3 where
6.0P less than S
More specifically, the conductor occupation ratio R of a conductor pattern formed in the ineffective region is set low near the coil pattern, and set high apart from the coil pattern. In this case, the electrical characteristics and mechanical strength of the coil can be consistent with each other. If necessary, a dummy pattern not serving as a current supply path to the coil pattern is formed in the ineffective region such that the conductor occupation ratio R of the conductor pattern in the ineffective region simultaneously satisfies inequalities 1, 2, and 3. This will be explained in more detail.
If the area of a conductor pattern formed in the first region A1 of the ineffective region that is nearest to the coil pattern is large, a large electrostatic capacitance is generated between the coil pattern and the conductor pattern formed in the first region A1. Such large electrostatic capacitance decreases the change rate of a current supplied to the coil to decrease the magnetic field inversion speed in generating a magnetic field modulated by the magnetic head. As a result, an information signal becomes difficult to record at a high speed. In the manufacture of a coil or after long-term use, the insulation reliability between the conductor pattern formed in the first region A1 and the coil pattern degrades. To prevent this, no conductor pattern is formed or the conductor occupation ratio R of the conductor pattern is suppressed to 0.3 or less in the first region A1.
If the conductor pattern forms near the coil pattern a closed loop surrounding the coil pattern, a current (eddy current) reverse to the supply current to the coil pattern is induced in the conductor pattern in supplying a current to the coil pattern and generating a magnetic field modulated by the magnetic head. Consequently, the change of a magnetic field to be generated is canceled, failing in normal information signal recording. To prevent this, it is preferable that the conductor pattern in the first region A1 be discontinuously formed by dividing the conductor pattern into two or more in the spiral direction of the coil pattern, and all the divided conductor patterns have an interval of 0.2P or more. This suppresses generation of an eddy current in the conductor pattern.
The second region A2 is also a range where the influence of a magnetic field generated by supplying a current to the coil pattern is exerted, not to such an extent as the first region A1. If the conductor occupation ratio R of a conductor pattern formed in the second region A2 exceeds 0.8, a generated eddy current or the electrostatic capacitance with the coil pattern degrades coil characteristics. To prevent this, no conductor pattern is formed or the conductor occupation ratio R of the conductor pattern is suppressed to 0.8 or less in the second region A2, as represented by inequality 2.
In the third region A3, if the conductor occupation ratio R of the conductor pattern is 0.3 or less, no reinforcing effect is substantially attained. If the conductor occupation ratio R of a conductor pattern formed in the third region A3 is lower than 0.6 times the conductor occupation ratio of the coil pattern, the current density in plating is biased to concentrate a current on the coil pattern in manufacturing conductive and coil patterns by plating. The conductor pattern in the third region A3 becomes thinner than the coil pattern, so the coil pattern undesirably protrudes. To efficiently generate a magnetic field, the conductor occupation ratio of the coil pattern is desirably 0.5 or more. Hence, as represented by inequality 3, the conductor occupation ratio R of the conductor pattern is set to 0.3 less than Rxe2x89xa61 in the third region A3. This relaxes local concentration of the current density in plating, and averages the metal ion diffusion rate within the pattern. Accordingly, the film thickness of the conductor pattern formed by plating is made uniform to prevent the coil pattern from protruding.
Note that the conductor occupation ratio R is the ratio, to the total area of each region, of the total area of all conductor patterns including a dummy pattern formed in the region and a conductor pattern for connecting terminals to each other and a terminal to a coil pattern. When the region includes a portion where no conductor pattern can be formed, e.g., a hole formed in part of the coil, this area is not included in the total area of the region. If the ineffective region includes a portion where the width is partially equal to or smaller than the pitch P of the coil pattern and the conductor pattern is difficult to form, a conductor pattern need not always be formed at this portion.
The conductor pattern (dummy pattern) formed in the ineffective region may have an arbitrary shape. Especially when a linear, slit-like, dot-like, or polygonal conductor pattern is periodically laid out, the conductor occupation ratio R of the conductor pattern is averaged over each region to decrease the thermal expansion and contraction distributions. Thus, the flatness, warpage, and dimensional precision of a conductive circuit can be improved, and the mechanical strength can be reinforced. Also when plating is applied, the current density and ion diffusion rate are averaged to make the plating film thickness more uniform. The layout period (pitch) of such conductor pattern may be constant or random. By setting the period (pitch) to be equal to or more than the pitch P of the coil pattern and equal to or less than 5P, the film thickness can be made more uniform.
Since the peripheral edge of the coil (edge portion such as the outer edge of the coil or the peripheral edge of a hole formed in the coil) requires a sufficient mechanical strength, the conductor pattern (dummy pattern) is desirably formed along the peripheral edge of the coil. However, if the conductor pattern is laid out to form a closed loop at the peripheral edge of a hole formed in the inner portion of the coil pattern to insert a magnetic pole or a hole serving as a light-transmitting portion, an eddy current generated in the conductor pattern cancels a magnetic field to be generated. For this reason, at least a conductor pattern forming a closed loop is not laid out at the peripheral edge of the hole formed at the inner portion of the coil pattern.
It is preferable that the conductor pattern formed at the peripheral edge of the coil have a band shape, and its width be equal to or more than the pitch P of the coil pattern and equal to or less than 4P. A narrower conductor pattern does not substantially reinforce the peripheral edge; or a wider conductor pattern increases the diffusion rate of metal ions in a plating solution and increases the thickness to be much larger than the coil pattern in forming a conductor pattern by plating. If this band-like conductor pattern is formed to be coupled to another conductor pattern in the ineffective region, the coil is reinforced and made more flat. The conductor pattern need not always be formed even at the peripheral edge of the coil as far as the interval between this peripheral edge and the outer edge of the coil pattern is equal to or less than the pitch P of the coil pattern. In this manner, the conductor pattern along the peripheral edge of the coil need not always be completely continuous to form a closed loop, but may be partially disconnected.
A positioning portion such as a circular or oval hole or a recess formed in the outer periphery of the coil is formed in the ineffective region, and a conductor pattern is formed at the peripheral edge of the positioning portion. This increases the mechanical strength around the positioning portion. In the following description, this conductor pattern formed at the peripheral edge of the positioning portion will be called a xe2x80x9cguide patternxe2x80x9d. Forming the guide pattern prevents the coil from deforming in fitting the positioning portion of the coil on a locking member attached to another building member such as the slider of the magnetic head. The relative positional precision to the optical head can further increase.
The conductor pattern formed in the ineffective region dissipates heat generated by the coil pattern or core formed in the effective region, thereby preventing the temperature rise of the magnetic head. A magnetic head having a heat dissipation member in tight contact with the conductor pattern can obtain high heat dissipation efficiency.
In the ineffective region, a conductor pattern having an appropriate shape can be formed at an appropriate position in accordance with the purpose. If all conductor patterns are formed such that their conductor occupation ratios R satisfy inequalities 1, 2, and 3, the mechanical strength of the coil increases without degrading the electrical characteristics of the coil. All conductor patterns including the coil pattern have almost the same thickness, which prevents some of the conductor patterns from protruding from the coil surface. In bonding the upper surface of the coil to another member such as a core, the lower surface (surface facing the magneto-optical recording medium) of the coil does not protrude or incline. As a result, the coil can be arranged at high precision so as to satisfactorily decrease the distance between its lower surface and the surface of the magneto-optical recording medium in manufacturing a magnetic head. The magnetic field can be efficiently applied to the magneto-optical recording medium.
The present invention implements a magnetic head excellent in heat dissipation characteristics which can increase the relative positional precision between the coil and the optical head and the distance precision from the magneto-optical recording medium, while the coil is downsized to reduce its inductance. This allows setting the magnetic field modulation frequency to 8 MHz or more, and increasing the information signal recording speed.
A flat coil component for the magnetic head according to the present invention can be manufactured by a combination of pattern formation and etching by photolithography, plating, and the like. In particular, when the present invention is applied to a coil formed from a conductor pattern having a thickness larger than the width of the coil pattern, i.e., having a high aspect ratio and a large film thickness, photolithography using a liquid photosensitive resin is optimum. That is, a thick resin setting pattern having a high aspect ratio is formed using a liquid photosensitive resin, and a conductor pattern as a conductive material film is formed as almost thick as the resin setting pattern by plating. If the set substance of the liquid photosensitive resin is not removed but is used as an insulating member, a flat coil component for a magnetic head can be manufactured in which the conductor pattern is as almost thick as the insulating member, which prevents the conductor pattern from protruding. Alternately, the set substance of the liquid photosensitive resin may be removed, and then an insulating member made of a thermosetting resin or the like may be buried to almost the same thickness as the conductor pattern. The insulating member may be formed thicker than the conductor pattern so as to cover the end face of the conductor pattern. Especially when the insulating member is formed as thick as or thicker than the conductor pattern on the upper surface side of the coil that is bonded to another member, the conductor pattern does not protrude, and the coil surface becomes very flat. This further prevents the lower surface of the coil from protruding or inclining upon bonding.